Chemistry Reference
In-Depth Information
these regions have shown actual declines at the tree line, with the biggest
decline 200 m below the tree line.
73
Although many species have been observed to move uphill in response to
temperature, this change alone is not enough to understand future plant
distributions. There is evidence of some downhill shifts despite climate
warming.
74-76
Drivers of species distribution shifts and mechanisms are not
well understood at present, but it's possible that some species may track
decreases in water deficit to lower elevations rather than temperature increases
to higher elevations.
77
In the past century there has been an increased density
of younger tree cohorts at lower elevations.
78,79
d
n
1
r
2
n
g
|
8
3.1.4 Alpine Forests
Climate projections show the greatest and earliest warming trends at higher
latitudes (45 to 65 uN), especially in continental interiors.
80,81
At the thermal
ecotone, small changes in temperature can have large consequences.
82
The
interior regions of Alaska are good places to investigate possible climate-
change effects on vegetation composition and soil thermal dynamics, because
changes in snowmelt affect surface and sub-surface soil moisture through
interactions with permafrost.
83
Changes in surface hydrology and soil
temperature affect forest and tundra vegetation as well as forage availability
for ungulates.
84
In Alaska, the tree line is shifting northward into the tundra,
important migratory bird and caribou habitat.
15
Yellow cedar (Callitropsis nootkatensis), a tree found in British Columbia,
Canada, and the Pacific Northwest of the USA, is experiencing varied
responses to changes in soil temperature and hydrological conditions.
85
Between the 19
th
century and the present, yellow cedar has seen a high rate of
mortality only in SE Alaska and bordering regions of Canada, with dieback
symptoms originating in the roots, spreading to the crown and finally
manifesting in the bole of the trees.
86
The emergence of the initial symptoms in
the roots indicates soil factors in the causes of stress,
85
which generally makes
trees more susceptible to predators; however, biotic agents are not the primary
cause of death.
87-93
Yellow cedar is usually drought tolerant and successful in poor soils,
94
but it
also tends to decline on poorly-drained soils,
87
and in the late 19
th
century a
warming period at high latitudes
95
was associated with yellow cedar decline.
96
D'Amore and Hennon
87
explored the soil conditions connected to cedar
decline and concluded that warmer air and soil conditions, reduced snow
packs, and early spring warming are causing trees to de-harden too early and
to become susceptible to late-frost injury.
3.2 Hydrologic Responses
Changes in temperature, precipitation and vegetation cover have major
implications for the global water balance in which soils play an integral role.
